We demonstrate a new method for measuring the sedimentation of a single colloidal bead by using a combination of optical tweezers and a stereo microscope based on a spatial light modulator. We use optical tweezers to raise a micron-sized silica bead to a fixed height and then release it to observe its 3D motion while it sediments under gravity. This experimental procedure provides two independent measurements of bead diameter and a measure of Faxén's correction, where the motion changes due to presence of the boundary.

Abstract:High-speed video stereo-microscopy relies on illumination from two distinct angles to create two views of a sample from different directions. The 3D trajectory of a microscopic object can then be reconstructed using parallax to combine 2D measurements of its position in each image. In this work, we evaluate the accuracy of 3D particle tracking using this technique, by extending the number of views from two to four directions. This allows us to record two independent sets of measurements of the 3D coordinates of tracked objects, and comparison of these enables measurement and minimisation of the tracking error in all dimensions. We demonstrate the method by tracking the motion of an optically trapped micro-sphere of 5 µm in diameter, and find an accuracy of 2-5 nm laterally, and 5-10 nm axially, representing a relative error of less than 2.5% of its range of motion in each dimension. Simpson, "An optically actuated surface scanning probe," Opt. Express 20, 29679-29693 (2012). 12. C. Alpmann, R. Bowman, M. Woerdemann, M. Padgett, and C. Denz, "Mathieu beams as versatile light moulds for 3d micro particle assemblies," Opt. Express 18, 26084-26091 (2010